Today, adhesive-coated plastic films, especially vinyl films with pressure sensitive adhesives or pressure-activated adhesives, are applied to a variety of surfaces for a variety of reasons such as advertisement, decoration, protection, and the like. Many of those surfaces contain rivets and other protrusions or indentations, such as the side of a truck trailer. When the film is applied over and adhered to these irregular surfaces, the film is strained to bring the adhesive into contact with the irregular surface. Residual stress in the film at such irregular surface locations often exceeds the holding power of the adhesive resulting in the film lifting off the surface to which it was adhered, particularly where the surface is irregular such as around a rivet or rib reinforcing the side of a truck trailer.
Current techniques for application to irregular surfaces involve applying most of the film with a small, plastic squeegee leaving a small area around the protrusion or indentation. Completing the application involves treating rivets to minimize lifting by heating the film with a heat source, usually a hot air gun or a torch, after the film has been predominately adhered to the irregular substrate. The film is typically heated while it is bridging the area around each type of surface irregularity, which can be summarized to be either a protrusion or an indentation. The film is not touched with the current tools because it is very soft and somewhat sticky. If it is touched, it is usually damaged. Because of the low mass of the film and the high temperature of the heat source, heating rates are several hundred degrees Celsius per second. Similar cooling rates are also occurring. When the film is then pushed into place with a tool, typically a squeegee for a rib or a rivet brush for a rivet, it is only slightly above room temperature. This does offer and improvement over pressing the film without warming because the delayed crystallization time of the film makes the film more compliant. A rivet brush is usually a stiff brush, usually about 2.54 cm in diameter with 1.25 cm long bristles attached to a short wooden handle. If the film is too soft because of the heating when it is contacted with a circular motion using the rivet brush, the film is likely to be damaged. If the film is too cool, the stress is not eliminated adequately, lifting results eventually. In an attempt to relax the residual stress, the film is often heated after application, but the temperature to which the film can be raised is limited by the thermal conductivity of the metal surfaces underneath the film. It is therefore very difficult for one skilled in the art to assuredly adhere the adhesive-coated film to the irregular surface while the film is fully softened without also damaging the film structure or its appearance. If there is damage, the film is weakened at that location and diminishes the durability of the film. If there is an image graphic on that film, the image is distorted or destroyed at the damaged location. An aberration in an image, even if the image is as large as a mural on the side of truck trailer, is quite noticeable and unsatisfactory to the owner of the trailer, the marketer of a product shown in the mural on the trailer, and the graphic fabricator who has invested considerable labor and other effort to adhere the graphic film to the side of the trailer.
If the film lifts because of residual stress, the film could crack, peel back, or be damaged and otherwise not meet expectations for a surface that should have paint-like appearance.